Authorities were given hope on Saturday night when they heard people yelling from the darkness. But rescuers had to turn back with conditions becoming more dangerous, and when they returned next day there were no signs of life.

"The mud was just too thick and deep," Snohomish County Fire District 21 Chief Travis Hots said at a news conference Sunday. "We didn't find anybody alive."

Five bodies were found Sunday, bringing the death toll to eight. Authorities said the number of people missing was 18 but Hots described this figure as "fluid."

In this aerial photo taken Saturday, March 23, 2014, a massive mudslide is shown in between the towns of Darrington, Wash., and Arlington, Wash. Marcus Yam / The Seattle Times via AP

The slide at around 11 a.m. locally also injured several people, including a 1-year-old infant.

The sheer depth of the mud made many conventional search techniques useless. "Thermal-imaging cameras are generally effective on a surface area. If someone’s buried, that won’t be effective," Hots said.

Rescuers who tried to reach affected homes on Saturday "got dragged in up to their armpits and had to be dragged out by ropes themselves," said Gov. Inslee during a briefing Sunday. Inslee had declared a state of emergency Saturday.

Two helicopters were deployed to search for victims, in hopes that rescuers would be able to trudge through the hazardous conditions to free them, Hots said.

Aerial Map of Snohomish landslide area.WSDOT

Another risk to the 100 rescuers was the potential overflow of the Stillaguamish River, which has been blocked by the slide and was rising 10-12 inches every half hour, according to John Pennington, director of the Snohomish County Department of Emergency Management.

The river's dam breached "slightly" Sunday afternoon, according to the Snohomish County Sheriff's Office, but officials assured the "flow won't be catastrophic."

The cause of the slide appears to be that the ground was over-saturated by heavy rainfall, the Snohomish County government said in a statement.